A new interferometer holds its atoms

Conventional atom interferometers measure gravity by throwing atoms upward and watching them fall. Light pulses tuned to particular resonance frequencies in the atoms deliver momentum kicks that split the atoms into two wavepackets, send the wavepackets along separate paths, and then recombine them. At a detection port, the matter waves interfere according to the phase difference between the two wavepackets.

But that approach has two limitations. An atomic fountain takes up a lot of space: The atoms are launched to a height of several meters in a vacuum. Even with such heights, the duration of the atoms’ free fall, which determines the interferometer’s sensitivity, is typically no more than a second or two. Holger Müller and his group at the University of California, Berkeley, have now demonstrated a method that extends the interrogation time to as long as 20 seconds. The achievement came from holding the two wavepackets in an optical lattice after the matter waves were split and separated by light pulses. Without the lattice to hold the atoms against gravity, interrogating them for that long in free fall would require a vacuum system a half kilometer tall. With the lattice, the system occupies just 1 meter of vertical space in the lab, which makes it attractive as a potentially mobile atomic gravimeter that can take data in the field.

The figure illustrates how the interferometer works. To make a gravity measurement, a cloud of ultracold atoms are launched upward a few millimeters into the center of an optical cavity. Two pairs of atomic beamsplitter (π/2) pulses separate the cloud into four distinct wavepackets that take four potential paths. Two of those are blocked, and two others rise in the same ground state g₂ and with the same momentum p+2ℏk. At the wavepackets’ apex, the optical lattice is turned on and holds them in a coherent quantum superposition. While suspended for a time t, only their height difference of 4 µm in a gravitational field distinguishes the wavepackets. When the lattice is turned off, a second pair of π/2 pulses recombine them so that they interfere and are detected.

Over 20 seconds, megaradians of phase difference accumulate between the two wavepackets. The optical cavity is crucial for such long coherence times. It spatially filters the trapping light because only the fundamental Gaussian mode of the cavity is resonant with the light. The many reflections keeps the optical lattice sites almost completely uniform and the superposition coherent. Indeed, the lasers so gently trap the wavepackets that they never get excited above the ground state. What’s more, the longer the hold time, the more the mechanical vibrations average out. (V. Xu et al., Science 366, 745, 2019 .)